In a laser scanner used in a conventional electrophotographic image forming apparatus, generally, an electrostatic latent image is formed by emitting a laser beam generated in correspondence with image data on a rotating polygon mirror and scanning the laser beam on an photosensitive member.
However, to realize high-speed and/or high-resolution image formation required in recent years, it is necessary to accelerate the rotation of the polygon mirror to determine the number of subscannings. However, as the rotation speed of polygon mirror has already been increased to almost a physical limitation, it is difficult to further increase the speed dramatically.
To address the above problem, provided is a laser scanner in which a mirror formed by e.g. the MEMS technique, employed in place of the polygon mirror, is torsionally reciprocated. In the scanner, a laser beam is emitted on this mirror and scanned on an photosensitive member.
FIG. 7 shows a schematic structure of the conventional laser scanner.
A laser beam 403 emitted from a laser driver 401 in correspondence with an input image signal (not shown) is reflected with a torsion mirror 402 which is torsionally reciprocated, thereby scanned on a rotating electrostatic drum 104, and forms an electrostatic latent image.
In this case, the torsion mirror 402 operates at a phase angle 602 of torsion movement in FIG. 8. As the torsion mirror 402 conducts an equiangular velocity movement, image formation by emission of laser beam cannot be performed in the whole period, but laser emission is performed only during an ENB (Enable) period 601. As a result, a scanning line 603 is drawn on the electrostatic drum.
FIG. 9 shows scanning of the laser beam on an electrostatic surface. Reference numeral 503 denotes a main scanning direction by the torsion movement of the torsion mirror; and 504, a subscanning direction by rotation of the electrostatic drum. As the laser beam is emitted only during the ENB period 601 corresponding to the forward movement of the torsion mirror, an electrostatic latent image is formed only with a scanning line 501 on the electrostatic drum, and laser emission is not performed in a portion indicated with a broken line 502 corresponding to the backward movement of the torsion mirror. If image formation is performed by emitting laser also in the portion 502, a normal image cannot be obtained since the angle of the scanning line is alternately changed.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-267995
However, in the above conventional structure, as image formation cannot be performed in the portion corresponding to the backward movement of the torsion mirror, the resolution is lowered. To raise the resolution, it is necessary to reduce the speed of the subscanning. Otherwise, complicated control is required so as to attain a high speed in the backward movement of the torsion mirror in comparison with that in the forward movement.